This invention relates to organic electroluminescent devices and more specifically to light-emitting layer compositions.
Light emitting diode (LED) technology is expected to be a major opportunity for advanced materials development impacting a large number of future technology based applications. These include flat panel displays which offer significant advantages over liquid crystal displays (LCDs) including much lower power requirements, improved definition, broader viewing angles and faster response times. The technology for LEDs offers the potential for lower cost lighting sources compared to incandescent lighting as well as fluorescent lighting applications. Inorganic based LEDs are already replacing some of these conventional applications including traffic lighting as well as flashlights offering equal or improved lighting at much lower power requirements.
Small molecule organic light emitting diodes (SMOLEDs) are being commercialized to replace LCD displays based on lower power requirements, faster response times, better definition and also easier fabrication. Such SMOLEDs are expected to revolutionize the flat panel display technology. Another area receiving considerable interest involves polymeric light emitting diodes (PLEDs) where polymeric light emitting materials can be utilized for flexible organic light emitting diodes (FOLEDs). A significant advantage of polymeric materials involves the fabrication possibilities. FOLEDs offer the potential for ink-jet printing of flat panel displays on flexible substrates such as indium-tin oxide coated polymeric films (i.e. poly(ethylene terephthalate)(PET), oriented polypropylene or polycarbonate). Roll to roll printing processes could also be utilized for FOLEDs. The potential for FOLEDs is considered to be quite large offering unique flat or contoured display panels. These FOLEDs may be of interest for unique lighting applications and large screen displays. These displays would be low cost, easy to install, very thin and power efficient. An example could be a battery operated TV screen, which would be the thickness of several sheets of paper and capable of folding, at a cost commensurate with the fabrication simplicity. Of course many problems have to be solved before these possibilities become reality.
Development of PLEDs has focused on polymeric materials which exhibit electroluminescence. These materials are generally conjugated polymers, such as poly(phenylene vinylene), polyfluorenes, polyphenylenes, polythiophenes and combinations of such structures. Conjugated polymers for use in PLEDs are disclosed by a number of references. See, e.g., U.S. Pat. No. 5,247,190 to Friend et al., U.S. Pat. No. 5,900,327 to Pei et al. and Andersson et al., J. Mater Chem., 9,1933–1940 (1999).
Variations of conjugated polymers useful for PLEDs include polymers comprised of oligomeric units of conjugated structures coupled into a high molecular weight polymer. See, e.g., U.S. Pat. No. 5,376,456 to Cumming et al., U.S. Pat. No. 5,609,970 to Kolb et al., Pinto et al., Polymer, 41, 2603–2611 (2000) and U.S. Pat. No. 6,030,550 to Angelopoulos et al.
A large number of low molecular weight compounds exist which exhibit fluorescence and electroluminescence. Some of these materials are commonly referred to as laser dyes. Many of these compounds offer very high fluorescence and thus electroluminescence. However, the properties desired for LED applications are generally only observed in solution or at low levels of doping in electro-optical or electroactive polymers. In the solid state, these materials can crystallize and lack the mechanical integrity to be utilized in PLEDs or SMOLEDs. Additionally (and more importantly), the excellent fluorescence and electroluminescence is lost with crystallization. These problems have been well documented in various reviews on the subjects of materials for LEDs. See, e.g., Kelly, “Flat Panel Displays. Advanced Organic Materials.” (Royal Society of Chemistry, 2000) at pp.155 and 177. Consequently, a number of attempts have been made to solve these problems.
For example, U.S. Pat. No. 6,329,082 to Kreuder et al. discloses hetero-spiro compounds suitable for use in LED devices. The compounds purportedly overcome “the unsatisfactory film-forming properties and . . . pronounced tendency to crystallize” of conventional low molecular weight fluorescent materials.
U.S. Pat. No. 6,214,481 to Sakai et al. purports to address problems with low emission intensity in solution and thermal instability of OLEDs by providing an organic host compound (e.g., distyrylarylene derivatives) for a fluorescent substance, wherein the host compound has a fluorescent quantum efficiency of at least 0.3 in a solid state and a Tg of at least 75° C.
Examples exist where fluorescent dopants are included in electroactive components of LEDs. See, e.g., Shoustikov et al., IEEE Journal of Selected Topics in Quantum Electronics, Vol. 4, No. 1 (1998), Djurovich et al., Polymer Preprints, 41(1), 770 (2000), Chen et al., Polymer Preprints 41(1), 835 (2000), U.S. Pat. No. 6,303,239 to Arai, U.S. Pat. No. 4,769,292 to Tang et al., U.S. Pat. No. 6,329,086 to Shi et al., U.S. Pat. No. 5,928,802 to Shi et al., and Hu et al., J. Appl. Phys., 83(11) 6002 (1998).
Examples also exist in the literature where fluorescent dyes have been added to non-active polymers for various applications. See, e.g., Quaranta et al., Synthetic Metals, 124, 75–77 (2001), Muller et al., Polymer Preprints, 41(1), 810 (2000), Sisk et al., Chemical Innovation, May 2000, U.S. Pat. No. 6,067,186 to Dalton et al., Kocher et al., Advanced Functional Materials, 11 (1), 31 (2001) and U.S. Pat. No. 5,952,778 to Haskal et al.
There are a number of examples in the literature where non-active polymers have been modified by side chain or main chain incorporation of optically active species. See, e.g., Hwang et al., Polymer, 41, 6581–6587 (2000), U.S. Pat. No. 5,414,069 to Cumming et al., U.S. Pat. No. 6,103,446 to Devlin et al., and U.S. patent application Publication US 2001/0026879 A1 to Chen et al.
U.S. Pat. No. 6,277,504 to Koch et al. discusses an electroluminescent assembly comprising a component which is a substituted or unsubstituted 1,3,5-tris(aminophenyl)benzene and a luminescent compound based on substituted metal complexed hydroxyquinoline compounds. The electroluminescent assembly can further comprise a polymeric binder. Similarly, U.S. Pat. No. 6,294,273 to Heuer et al. discloses a polymeric binder for the electroluminescent compound of a metal complex of N-alkyl-2,2′-imino-bis(8-hydroxy-quinoline).
Various references note blends of active electroluminescent polymers for utility in LED devices offering in many cases improved performance over the individual constituents. See, e.g., Hu et al., J. Appl. Phys., 76(4), 2419 (1994), and Yang et al., Macromol. Symp., 124, 83–87 (1997).
Blends of fluorene-based alternating polymer with non-active polymers (e.g. PMMA, epoxy resin, polystyrene) are disclosed in U.S. Pat. No. 5,876,864 to Kim et al. U.S. Pat. No. 6,255,449 to Woo et al. notes the utility of blends of specific fluorene containing polymers and a litany of other polymers, including conjugated polymers.
Frederiksen et al., J. Mater. Chem., 4(5), 675–678 (1994) teaches the addition of laser dyes to a polystyrene matrix for use in a LED device.
U.S. Pat. No. 5,821,003 to Uemura et al. notes the use of polymeric binders for low molecular weight hole transport materials for the hole transport layer of LED devices. Examples include polysulfone and aromatic tertiary amines. The inclusion of minor amounts of fluorescent compounds in the polymer bound hole transport layer is noted to improve the luminance of blue and white.
U.S. Pat. No. 5,663,573 discloses the use of a variety of organic light emitting materials for preparing a bipolar electroluminescent device, including polypyridines, polypyridylvinylenes, polythiophenes, polyphenylenes, polyphenylenevinylenes, polyphenylenebenzobisthiazoles, polybenzimidazobenzophenanthrolines, polyfluorenes, polyvinylcarbazoles, polynaphthalenevinylenes, polythienylenevinylenes, polyphenylene-acetylenes, polyphenylenediacetylenes and polycyanoterephthalylidenes.
Despite the foregoing developments, it is desired to incorporate the excellent properties of low molecular weight electroluminescent materials such as laser dyes as a material which could be utilized in a LED device with fabrication characteristics typically exhibited by PLEDs and the crystallization behavior characteristic of these materials effectively eliminated.
All references cited herein are incorporated herein by reference in their entireties.